The invention is in the field of transistor amplifier circuits, and relates more particularly to a power amplifier circuit having a bias boosting circuit for increasing maximum power output and reducing power dissipation at low power levels.
Amplifiers of this general type are frequently used in high-frequency RF amplifiers, as well as in audio amplifiers and other applications. In order to obtain a linear input-output relationship and high operating efficiency, such amplifiers are typically operated with a conduction angle of about 180.degree. (Class B) or slightly greater (Class AB) to avoid crossover distortion.
Typically, amplifiers of this type require a dc bias circuit to establish the quiescent bias current in the amplifier circuit to ensure operation in the Class B or Class AB mode. In the prior art, bias is typically provided by a fixed current source, as shown in U.S. Pat. No. 5,844,443, or else by an external supply, which can be set to a desired constant value to secure the quiescent current necessary to operate in the desired mode, as shown in U.S. Pat. No. 5,548,248.
However, in amplifiers of the type described above the average current drawn from the supply depends upon the input signal level. As the output power increases so does the average current in both the emitter and the base of the power transistor. This increased average current causes an increased voltage drop in the biasing circuitry and in ballast resistors (which are used to avoid hot-spotting and thermal runaway in transistors using an interdigitated design). This in turn reduces the conduction angle (i.e. the number of degrees out of 360.degree. that the amplifier is conducting), and forces the amplifier deep into Class B or even Class C operation, thereby reducing the maximum power output by about 25%. To avoid this power reduction, the amplifier must have a larger quiescent bias. In prior-art circuitry this inevitably leads to a higher power dissipation at low power output levels and therefore an undesirable tradeoff in operating characteristics.
Accordingly, it would be desirable to have a power amplifier circuit which offers the advantages of optimum maximum output power and reduced power dissipation at low power levels. Additionally, it would be desirable for such a circuit to be simple and compact in design, and economical to manufacture.